Royal Society - David Keith

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Introduction
1.1 Background
Geoengineering, or the deliberate large-scale manipulation
of the planetary environment to counteract anthropogenic
climate change, has been suggested as a new potential
tool for addressing climate change. Efforts to address
climate change have primarily focused on mitigation, the
reduction of greenhouse gas emissions, and more recently
on addressing the impacts of climate change—adaptation.
However, international political consensus on the need to
reduce emissions has been very slow in coming, and there
is as yet no agreement on the emissions reductions needed
beyond 2012. As a result global emissions have continued
to increase by about 3% per year (Raupach et al. 2007),
a faster rate than that projected by the Intergovernmental
Panel on Climate Change (IPCC) (IPCC 2001) 7 even under
its most fossil fuel intensive scenario (A1FI 8 ) in which an
increase in global mean temperature of about 4°C (2.4 to
6.4°C) by 2100 is projected (Rahmstorf et al. 2007).
The scientific community is now becoming increasingly
concerned that emissions will not be reduced at the rate
and magnitude required to keep the increase in global
average temperature below 2°C (above pre-industrial
levels) by 2100. Concerns with the lack of progress of
the political processes have led to increasing interest in
geoengineering approaches. This Royal Society report
presents an independent scientific review of the range
of methods proposed with the aim of providing an
objective view on whether geoengineering could, and
should, play a role in addressing climate change, and
under what conditions.
1.2 Geoengineering
Geoengineering proposals aim to intervene in the climate
system by deliberately modifying the Earth’s energy
balance to reduce increases of temperature and eventually
stabilise temperature at a lower level than would otherwise
be attained (see Figure 1.1). The methods proposed are
diverse and vary greatly in terms of their technological
characteristics and possible consequences. In this report
they have been classified into two main groups:
i. Carbon dioxide removal (CDR) methods: which reduce
the levels of carbon dioxide (CO 2 ) in the atmosphere,
allowing outgoing long-wave (thermal infra-red) heat
radiation to escape more easily;
7 Because of the economic crisis, 2008 and 2009 emissions will be
lower than the most pessimistic of the IPCC Special Report on
Emissions Scenarios (SRES). However, this emission reduction is due
only to the downturn in GDP growth. Underlying factors, such as rates
of deployment of carbon-neutral energy sources and improvement in
efficiency continue to be worse than even the most pessimistic of the
IPCC emission scenarios.
8 The A1FI scenario is based on a future world of very rapid economic
growth, a global population that peaks in mid-century and declines
thereafter, and the rapid introduction of new and more efficient (but
fossil fuel intensive) technologies (IPCC 2000a).
or:
ii. Solar radiation management (SRM) methods: which
reduce the net incoming short-wave (ultra-violet and
visible) solar radiation received, by deflecting sunlight,
or by increasing the reflectivity (albedo) of the
atmosphere, clouds or the Earth’s surface.
Note that while it would theoretically also be possible for
geoengineering methods to remove greenhouse gases
other than CO 2 from the atmosphere (eg, methane (CH 4 ),
nitrous oxide (N 2 O)), most if not all of the methods
proposed so far focus on CO 2 which is long-lived, and
present at a relatively high concentration, and so these are
the focus in this report. Mitigation efforts to reduce
emissions of such non-CO 2 greenhouse gases are of
course still extremely important, but are not regarded as
geoengineering and so are not considered.
The objective of CDR methods is to remove CO 2 from the
atmosphere by:
• Enhancing uptake and storage by terrestrial biological
systems;
• Enhancing uptake and storage by oceanic biological
systems; or
• Using engineered systems (physical, chemical,
biochemical).
SRM methods may be:
• Surface-based (land or ocean albedo modification);
• Troposphere-based (cloud modification methods, etc.);
• Upper atmosphere-based (tropopause and above,
ie, stratosphere, mesosphere);
• Space-based.
1.3 The climate system
To understand the principles of geoengineering and the
methods by which the range of interventions have effect it
is necessary to understand the climate system. A detailed
review of the science of climate change is provided in the
IPCC Fourth Assessment working group 1 report (AR4)
(IPCC 2007a). Here brief descriptions of the climate system
and the drivers that lead to climate change are provided.
Most geoengineering proposals aim either to reduce the
concentration of CO 2 in the atmosphere (CDR techniques,
Chapter 2), or to prevent the Earth from absorbing some
solar radiation, either by deflecting it in space before it
reaches the planet, or by increasing the reflectivity of the
Earth’s surface or atmosphere (SRM techniques, Chapter 3).
These geoengineering techniques would work by
manipulating the energy balance of the Earth: the balance
between incoming radiation from the sun (mainly short-wave
ultraviolet and visible light) that acts to heat the Earth, and
The Royal Society
Geoengineering the Climate I September 2009 I 1